Vaccination plays an important role in preventing infectious diseases. For example, vaccination strategies have proven successful in eradicating smallpox and in diminishing the incidence of many other infectious diseases worldwide. Traditional vaccination methods employ pathogens' antigens as immunogens. In one traditional approach, antigens of infectious organisms are introduced into a host. In a variation of this approach, DNA encoding an antigen of an infectious organism is introduced into a host, and the host makes antigens to which the immune system responds.
The fixed nature of a traditional vaccine can be a major limitation to its success in preventing diseases caused by mutable pathogens (e.g., influenza, HIV, hepatitis C). When pathogens mutate to form antigenic variants, an immune response directed to non-mutated organisms can be obsolete and ineffective. Because immune responses typically develop much more slowly than new variants proliferate, mutable pathogens can rapidly spread through populations.
The current strategy used to prevent infectious diseases caused by mutable pathogens is to vaccinate repeatedly, each time a new strain is identified. The current approach has several drawbacks. First, since the nature of spontaneous variants cannot be predicted, repeated vaccination cannot proactively protect against these variants. Second, repeated vaccination requires frequent administration and raises concerns about compliance. Third, extensive vaccination of the population for new pathogen variants increases the selection pressure for more virulent mutants and could lead to pandemics.